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Prevent concurrent SimpleLruTruncate() for any given SLRU.

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The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com
This commit is contained in:
Noah Misch
2020-08-15 10:15:53 -07:00
parent b538e83f17
commit 592a589a04
11 changed files with 117 additions and 13 deletions
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4
doc/src/sgml/catalogs.sgml
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@ -10215,7 +10215,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
and general database objects (identified by class OID and object OID,
in the same way as in <structname>pg_description</structname> or
<structname>pg_depend</structname>). Also, the right to extend a
relation is represented as a separate lockable object.
relation is represented as a separate lockable object, as is the right to
update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>.
Also, <quote>advisory</quote> locks can be taken on numbers that have
user-defined meanings.
</para>
@ -10243,6 +10244,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
Type of the lockable object:
<literal>relation</literal>,
<literal>extend</literal>,
<literal>frozenid</literal>,
<literal>page</literal>,
<literal>tuple</literal>,
<literal>transactionid</literal>,

16
doc/src/sgml/monitoring.sgml
View File

@ -1738,6 +1738,12 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry><literal>extend</literal></entry>
<entry>Waiting to extend a relation.</entry>
</row>
<row>
<entry><literal>frozenid</literal></entry>
<entry>Waiting to
update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>
and <structname>pg_database</structname>.<structfield>datminmxid</structfield>.</entry>
</row>
<row>
<entry><literal>object</literal></entry>
<entry>Waiting to acquire a lock on a non-relation database object.</entry>
@ -1906,6 +1912,11 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry><literal>NotifyQueue</literal></entry>
<entry>Waiting to read or update <command>NOTIFY</command> messages.</entry>
</row>
<row>
<entry><literal>NotifyQueueTail</literal></entry>
<entry>Waiting to update limit on <command>NOTIFY</command> message
storage.</entry>
</row>
<row>
<entry><literal>NotifySLRU</literal></entry>
<entry>Waiting to access the <command>NOTIFY</command> message SLRU
@ -2082,6 +2093,11 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry><literal>WALWrite</literal></entry>
<entry>Waiting for WAL buffers to be written to disk.</entry>
</row>
<row>
<entry><literal>WrapLimitsVacuum</literal></entry>
<entry>Waiting to update limits on transaction id and multixact
consumption.</entry>
</row>
<row>
<entry><literal>XactBuffer</literal></entry>
<entry>Waiting for I/O on a transaction status SLRU buffer.</entry>